Power tools

ABSTRACT

Power tool  1  may include table  5  on which work W is positioned. A portion of a circular blade  3  protrudes above table  5 . Circular blade  3  may be driven by a motor. The motor may be controlled by a control device  90 . Work W is cut by means of an operator sending work W positioned on an upper face of table  5  in the direction of the circular blade  3  while circular blade  3  is being driven by the motor. Power tool  1  may include first radar device  86  and second radar device  87  for monitoring a predetermined area in the vicinity of circular blade  3 . First radar device  86  may detect whether objects other than work are present in the vicinity of a outer edge of circular blade  3 . Second radar device  87  may detect the location of objects moving within the predetermined area in the vicinity of circular blade and detects the speed at which the objects are moving in the direction in which work is sent. Control device  90  may cause an emergency halt of the motor in the case where first radar device  86  detects that an object other than work is present in the vicinity of the outer edge of circular blade  3 . Further, Control device  90  may cause an emergency halt of the motor in the case where an object detected by second radar device  87  has a predetermined positional relationship relative to circular blade  3  and the detected speed exceeds a predetermined value.

CROSS REFERENCE

This application claims priority to Japanese patent application number2002-328837, filed Nov. 12, 2002, and Japanese patent application number2003-81399, filed Mar. 24, 2003, each of which are incorporated hereinby reference as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power tools, e.g., table saws, mitersaws and the like. Specifically, techniques are described for preventinga cutting tool from making contact with objects other than work.

2. Description of the Related Art

U.S. unexamined patent application no. 17336/2002 describes a power toolthat carries out an emergency stop when a cutting tool has made contactwith a person (i.e., an object other than work). The known power toolincludes a contact detection system that detects contact between aperson and a cutting tool. The contact detection system is electricallycoupled to the cutting tool and monitors an electrical potential of thecutting tool in order to detect contact between a person and the cuttingtool. If contact between the person and the cutting tool is detected bythe contact detection system, power supply to the motor is stopped,effecting an emergency stop of the cutting tool.

SUMMARY OF THE INVENTION

However, in the known power tool, movement of the cutting tool is haltedonly after contact between a person and the cutting tool has beendetected, and it is not possible to prevent contact between the personand the moving cutting tool.

It is, accordingly, one object of the present teachings to provideimproved power tools that can prevent a cutting tool from making contactwith objects other than work (e.g., persons, etc).

In one aspect of the present teachings, power tools are taught that arecapable of detecting abnormal conditions before contact between thecutting tool and objects other than work occurs. Therefore, if theabnormal conditions are detected, the power tools can warn operatorsand/or stop movement of the cutting tool.

Thus, in one embodiment of the present teachings, power tools mayinclude a cutting tool, such as a circular blade or saw blade, and adrive source, such as an electric motor, for driving the cutting tool.Such power tools may also include a detecting device (e.g., a detectingdevice using radio waves, a detecting device using ultrasonic waves, adetecting device using infrared rays, etc.) and a control device, suchas a microprocessor or processor, in communication with the detectingdevice. For example, the detecting device may detect the location andspeed of objects (e.g., work, etc.) moving within a predetermined areanear the cutting tool. On the basis of the location and speed of theobjects detected by the detecting device the control device maydetermine whether operating conditions are normal or abnormal. Forexample, the control device may determine whether the cutting tool andthe objects detected by the detecting device have a predeterminedpositional relationship (e.g., whether the distance between the cuttingtool and the object is within a predetermined value), and also determinewhether the speed of the objects detected by the detecting devicetowards the cutting tool exceeds a predetermined value. From the resultsof these determinations it may be decided whether operating conditionsare normal or abnormal. For example, when a detected object is moving atnormal speed near the cutting tool and in a direction approaching thecutting tool, it may be determined that this is simply work beingdelivered for cutting at a normal speed and that operating condition isnormal. However, when the detected object is moving at rapid speed nearthe cutting tool and in a direction approaching the cutting tool, it maybe determined that operating conditions is abnormal. Since it can bedetermined whether operating conditions are normal or abnormal beforecontact between the object and the cutting tool occurs, contact betweenthe object and the cutting tool can be prevented under abnormaloperating conditions.

When operating conditions have been determined to be abnormal, a warningmay be given to the power tool operator, and/or the movement of thecutting tool may be automatically stopped. For example, the power toolsmay also include a buzzer that generates a warning sound. Further, thepower tool may also include a switch for cutting off power supply to themotor. As another example, the power tool may also include a brakemechanism that engages and stop the cutting tool, or retractingmechanism that retract the cutting tool from its operating position.Further, the power tool may also include a barrier that is placedbetween the cutting tool and the operator when operating conditions havebeen determined to be abnormal.

Preferably, the detecting device may include a radar device thattransmits radio waves towards the predetermined area and receives theradio waves reflected therefrom. By using the radio waves, the locationand speed of the object can be detected accurately even if chips areformed during the cutting operation.

Further, it is preferred that the frequency of the radio wavestransmitted from the radar device is 1 GHz or above, and it is morepreferred that the frequency is in the range of 10˜30 GHz. By usingradio waves of this frequency, directivity can be improved, and it ispossible to monitor only the surroundings of the cutting tool.

In another embodiment of the present teachings, the power tools mayfurther include a table on an upper face of which the work ispositioned. A portion of the cutting tool may protrude above the table,this protruding portion cutting the work. In this case, the area to bemonitored by the radar device may be restricted to above the table. Forexample, it is possible to monitor only an area that rises to apredetermined height above the table and is within a predetermined rangeof distance from side faces of the cutting tool. Further, it ispreferred that the radar device is disposed in positions so as tosandwich the table and face towards a power tool operator. This type ofconfiguration prevents the radar device from obstructing the operationsof the power tool operator.

In another aspect of the present teachings, power tools may include acutting tool and a motor for driving the cutting tool. The power toolmay further include a radar device and a processor in communication withthe radar device. The radar device preferably transmits radio wavestowards a predetermined area in the vicinity of a contacting locationwhere an edge of the cutting tool and work make contact, and receivesradio waves reflected therefrom. The processor preferably determinesfrom the reflected radio waves received by the radar device whether anobject other than work is in the predetermined area. For example, usingthe difference between the waves reflected when work is in thepredetermined area and the waves reflected when an object other thanwork is in the predetermined area, the processor can determine whetherwork or an object other than work is in the predetermined area. When ithas been determined that an object other than work is in thepredetermined area, a warning may be given to the power tool operator,and/or the movement of the cutting tool may be immediately stopped. Bythis means, contact between the cutting tool and an object other thanwork can be prevented.

Preferably, the power tools may also include a memory for storing thereflected radio waves created when the work is disposed within thepredetermined area. The reflected waves can be stored as time seriesdata in the memory. Alternatively, only identification informationextracted from the reflected waves (e.g., peak values of the reflectedwaves, waveform patterns, etc.) may be stored. Further, the processormay determine whether an object other than work is in the predeterminedarea by using the reflected waves received by the radar device and thereflected waves stored in the memory. For example, the processorpreferably determines that an object other than work is in thepredetermined area when the absolute value of the difference between thepeak values of the reflected waves received by the radar device and peakvalues of the reflected waves stored in the memory exceeds apredetermined threshold value. Since the reflected waves created whenthe work is disposed in the predetermined area are already stored, thisconfiguration allows an accurate determination of whether an objectother than work is in the predetermined area.

Generally, the radio wave reflection coefficient of materials variesaccording to frequency. As a result the radio waves may be transmittedfrom the radar device as impulses (i.e., including many frequencyelements), and the processor may perform frequency analysis on thereflected waveforms to determine whether an object other than work ispresent within the predetermined area.

In the alternative, in the case where the work is wood, the radio wavereflection coefficient characteristics of wood can be taken into accountand only radio waves within a narrow frequency range can be transmitted(e.g., single frequency radio waves) to allow the determination ofwhether an object other than work is present within the predeterminedarea. For example, the frequency of the radio waves transmitted from theradar device may be established between the range of 1˜30 GHz. Radiowaves with a frequency of 1˜30 GHz have a low reflection coefficient forwooden material that has a low moisture content, and have a highreflection coefficient for objects with a high moisture content (e.g.,hands, fingers, etc.). Consequently, it is possible to identify whetherthe object from which the radio waves are reflected is work or an objectother than work (i.e., an object with a high moisture content) eventhough radio waves within a narrow frequency range are transmitted. Thatis, when the peak values of the reflected waves received by the radardevice exceed a predetermined threshold, it can be determined that anobject other than work is present in the predetermined area. Further,even in the case where the frequency of the radio waves is within therange of 1˜30 GHz, the frequency may be changed in accordance with one'saims. For example, it is preferred that a lower radio-wave frequency ischosen for penetrating bulky wood, and that a higher radio-wavefrequency is chosen for improving the directivity of the radio waves.

In another embodiment of the present teachings, the power tools mayfurther include a table on an upper face of which the work ispositioned. A portion of the cutting tool may protrude above the table,this protruding portion cutting the work. In this case, it is preferredthat the radar device may be disposed beneath the table and that thetable may have a penetrable window which can allow the radio wavestransmitted from the radar to penetrate therethrough. The penetrablewindow can be manufactured from a material (e.g., resin) through whichradio waves penetrate easily. Locating the radar device beneath thetable prevents the radar device from obstructing the operator.

In another embodiment of the present teachings, the power tools mayinclude a table on an upper face of which work is positioned, and an armslidably or pivotably attached to the table. A cutting area for cuttingthe work may be provided on the table. The cutting tool may be rotatablyattached to the arm. By moving the arm relative to the table, thecutting tool can be moved between an operating position close to thecutting area and a waiting position removed therefrom. In this case, itis preferred that the radar device transmits the radio waves towards thecutting area and receives the radio waves reflected therefrom.

In another aspect of the present teachings, the radar device may includea radio wave transmitting member and a radio wave receiving member.Preferably, at least one of the radio wave transmitting member and theradio wave receiving member may have a plurality of microstrip antennas.By using the microstrip antennas, the radio wave transmitting member orthe radio wave receiving member can be miniaturized and can save space.Further, by using a plurality of microstrip antennas or patch antennas(i.e., a type of microstrip antenna), the desired directivity can beobtained. Further, the radio wave transmitting member and the radio wavereceiving member may have different antennas. Alternatively, the radiowave transmitting member and the radio wave receiving member may havethe same antenna.

Preferably, the microstrip antenna may include a strip conductor, aground conductor disposed in a position opposite the strip conductor,and a dielectric layer disposed between the strip conductor and theground conductor. In this case, a groove may be formed in a surface ofthe dielectric layer and that the strip conductor may be disposed withinthe groove. Since the strip conductor does not protrude from the surfaceof the dielectric layer, damage to the strip conductor can be prevented.Further, a groove may be formed in the ground conductor and that thedielectric layer may disposed within the groove formed in the groundconductor. By this means, the dielectric layer does not protrude fromthe ground conductor, and consequently damage to the dielectric layercan be prevented. Preferably, the microstrip antenna may be disposedwithin a surface of a housing of the power tools (e.g., a table, etc.).

These aspects and features may be utilized singularly or, incombination, in order to make improved power tools, including but notlimited to, table saws, miter saws. In addition, other objects, featuresand advantages of the present teachings will be readily understood afterreading the following detailed description together with theaccompanying drawings and claims. Of course, the additional features andaspects disclosed herein also may be utilized singularly or, incombination with the above-described aspect and features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional side view showing a table sawaccording to a first representative embodiment of the present teachings.

FIG. 2 is a partial cross-sectional plane view of the table saw shown inFIG. 1.

FIG. 3 schematically shows the positional relationship between acircular blade and a penetrable window.

FIG. 4 is a block diagram schematically showing a representative circuitof a first radar device.

FIG. 5A schematically shows a waveform of an output gate signal of thefirst radar device.

FIG. 5B schematically shows a waveform of output signal from anoscillation circuit of the first radar device.

FIG. 5C schematically shows a waveform of a radio wave received by thefirst radar device when only wooden work is disposed in a firstpredetermined area.

FIG. 5D schematically shows a waveform of a radio wave received by thefirst radar device when work W and a finger are disposed in the firstpredetermined area.

FIG. 6 is a block diagram showing a representative circuit of a secondradar device.

FIG. 7 schematically shows the relationship between frequency and timeof radio waves transmitted from the second radar device.

FIG. 8 schematically shows an area monitored by the second radar device.

FIG. 9 is a block diagram showing a representative circuit of the tablesaw of the first embodiment.

FIG. 10 is a flowchart of a representative process for cutting a workusing the table saw.

FIG. 11 shows the positional relationship between the circular blade andthe area monitored by the second radar device divided into zone I, zoneII, and zone III.

FIG. 12A shows a representative example for disposing the second radardevice relative to the table saw of the first representative embodiment.

FIG. 12B shows another representative example for disposing the secondradar device relative to the table saw of the first representativeembodiment.

FIG. 12C shows another representative example for disposing the secondradar device relative to the table saw of the first representativeembodiment.

FIG. 13A shows a representative configuration of a microstrip antennaused in a table saw of a second representative embodiment of the presentteachings.

FIG. 13B shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 13C shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 13D shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 13E shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 13F shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 13G shows another representative configuration of a microstripantenna used in the table saw of the second representative embodiment ofthe present teachings.

FIG. 14 schematically shows plane and side views of the table saw of thesecond representative embodiment.

FIG. 15 is a cross-sectional view of an antenna member disposed within atable of the table saw shown in FIG. 14.

FIG. 16 schematically shows a representative example of an arrangementof patch antennas disposed within the table.

FIG. 17 schematically shows another representative example of anarrangement of patch antennas disposed within the table.

DETAILED DESCRIPTION OF THE INVENTION

First Detailed Representative Embodiment

FIG. 1 shows a first detailed representative embodiment of the presentteachings, which is table saw 1 having table 5 on which to positionwooden work W. A portion of circular blade 3 protrudes above table 5,and top and sides of this protruding portion are covered by blade guard7. Blade guard 7 is rotatably attached to table 5 and is pushed open bywork W during cutting.

As shown in FIGS. 1 and 2, a lower portion of circular blade 3 may bedisposed within blade hood 21 that is attached to table 5 in a mannerwhereby it can be inclined. Openings 81 and 82 for allowing motorhousing 23 to move up and down are formed in a side face of blade hood21. Motor housing 23 is attached, in a manner whereby up and downmovement is possible, to the side face of blade hood 21 via two guidebars 25 a and 25 b. Motor M is disposed within motor housing 23.Circular blade 3 is attached to a drive shaft of motor M.

As shown in FIG. 1, splitting blade 9 for preventing the closure of thehole cut in the work W by circular blade 3 may be attached at theposterior of circular blade 3. Splitting blade 9 is fixed to a posteriorend of motor housing 23 by means of bracket 27 fastened by bolts. Thus,as the height to which circular blade 3 is exposed above table 5 changesas motor housing 23 is moved up and down, splitting blade 9 moves up anddown therewith.

Next, the mechanism for moving motor housing 23 up and down will beexplained. Motor housing 23 may be moved up and down by means ofrotating handle 31 that projects at the anterior of table 5. Shaft 33 ofhandle 31 is the same axis as shaft 37 of inclining dial 35. Bevel gear39 is connected to a tip of shaft 33. Bevel gear 43 engages bevel gear39, bevel gear 43 being connected to a lower end of threaded shaft 41that extends in a longitudinal direction.

The upper and lower ends of threaded shaft 41 are fixed to blade hood21, threaded shaft 41 rotating in one spot without moving up or down. Anut member (not shown) having an inner thread is coupled to threadedshaft 41, and the nut member is fixed to motor housing 23. As a result,when handle 31 is rotated, motor housing 23 is moved up or down by meansof the thread feed mechanism of threaded shaft 41 and the nut member.Guide bars 25 a and 25 b function to guide the up-down movement of motorhousing 23.

Next, the mechanism for causing circular blade 3 to incline will beexplained. Blade hood 21 may be inclined by rotating inclining dial 35that has the same axis as handle 31. As shown in FIG. 2, plate 53 havingarc-shaped gear 51 fixed thereto is attached to an anterior side oftable 5. Arc-shaped slit 55 which follows arc-shaped gear 51 is formedin the plate 53. Shaft 33 of handle 31 passes through slit 55 to aninner side. Pinion gear 57 that engages arc-shaped gear 51 is fixed toshaft 37 of inclining dial 35. As a result, when inclining dial 35 isrotated, pinion gear 57 moves along the arc of arc-shaped gear 51, andblade hood 21 inclines therewith. When blade hood 21 has inclined suchthat circular blade 3 has reached a desired angle, locking lever 83 isoperated to fix blade hood 21.

As shown in FIG. 1, first radar device 86 and second radar device 87 maybe disposed at the anterior and posterior respectively of circular blade3. First radar device 86 may monitor a first predetermined area that isin the vicinity of a location where an outer edge of circular blade 3and work W make contact. As shown in FIG. 1, first radar device 86 isdisposed to the anterior of circular blade 3 below table 5. As shown inFIG. 3, table 5 may have penetrable window 5 a, through which radiowaves penetrate, near the anterior edge of circular blade 3. A platemade from resin may be utilized to form penetrable window 5 a.

Second radar device 87 may monitor a second predetermined area thatsurrounds the portion of circular blade 3 that protrudes above table 5.As shown in FIGS. 1 and 2, second radar device 87 may be attached to thetip of arm 85 attached to the posterior of table 5. As is clear from thefigures, second radar device 87 is disposed above and to the posteriorof circular blade 3.

Next, first radar device 86 and second radar device 87 will be explainedin more detail. First, first radar device 86 will be explained. FIG. 4is a block diagram showing a representative circuit of the first radar86. As shown in FIG. 4, first radar device may include antenna 124 fortransmitting and receiving radio waves. Oscillation circuit 122 foroscillating an electrical signal at a specified frequency and outputtingthis electrical signal may be connected to antenna 124 (specifically, toa radio wave transmitting member of antenna 124). Clock circuit 120 maybe connected to oscillation circuit 122. Clock circuit 120 is a circuitfor periodically causing the output of oscillation circuit 122 to be ONor OFF. Radio waves are transmitted from antenna 124 only while clockcircuit 120 causes the output of oscillation circuit 122 to be ON.

Waveform shaping circuit 132 may be connected to antenna 124(specifically, to a radio wave receiving member of antenna 124) viaamplifying circuit 128 and filter circuit 130. Amplifying circuit 128amplifies the signal of the radio waves received by antenna 124. Filtercircuit 130 filters noise from the signal amplified by amplifyingcircuit 128. Waveform shaping circuit 132 shapes the waveform of thesignal that was output from filter circuit 130, then outputs the shapedsignal to control device 90.

Preferably, microwaves (i.e., frequency: 3˜30 GHz) may be used in theradio waves that are output from first radar device 86; in the firstrepresentative embodiment, 10.5 GHz microwaves may be used. The radiowave reflection coefficient of wooden work W and the radio wavereflection coefficient of an object other than work (e.g., a operator'shand or finger, etc.) differ greatly with the radio waves of thisfrequency band, and this difference in radio wave reflectioncoefficients is utilized to enable discrimination between work W andobjects other than work W. Specifically, with radio waves of thisfrequency band, the radio wave reflection coefficient is low with wood,which has a low moisture content, and the radio wave reflectioncoefficient is high with objects having a high moisture content. As aresult, in the first representative embodiment, the strength of the peakvalues of the reflected waves are used to determine whether thereflected waves were reflected from work W or from an object other thanwork which was located above the work W.

FIGS. 5A˜5D shows radio waves transmitted from first radar device 86together with output waveforms of radio waves received by first radardevice 86. FIG. 5A shows the waveform of an output gate for outputtingthe signal of oscillation circuit 122 to antenna 124. FIG. 5B shows thewaveform of the signal that is actually being output from oscillationcircuit 122 to antenna 124. FIG. 5C shows the output waveform of a radiowave received by first radar device 86 when only wooden work W islocated in the first predetermined area. FIG. 5D shows the outputwaveform of a radio wave received by first radar device 86 when work Wand a finger are located in the first predetermined area.

As shown in FIG. 5A, the output gate for outputting the signal ofoscillation circuit 122 is ON only for periodic time intervals Tp. As aresult, as shown in FIG. 5B, a signal of 10.5 GHz is output fromoscillation circuit 122 only while the output gate is ON, radio wavesbeing transmitted from the radio wave transmitting member of antenna 124on the basis of this output signal. After the radio waves have beentransmitted from antenna 124, these transmitted radio waves and thereflected radio waves are received by the radio wave receiving member ofantenna 124. In FIGS. 5C and 5D, ‘a’ are waves that were transmittedfrom the radio wave transmitting member and received directly by theradio wave receiving member, ‘b’ and ‘d’ are reflected waves that werereflected from objects in the first predetermined area. As is clear fromthe figures, the reflected waves ‘b’ reflected from work W have a lowpeak voltage, whereas the reflected waves ‘d’ that penetrate work W andare reflected from a finger have a high peak voltage. Consequently, itis possible to determine, on the basis of the peak voltages of thereflected waves received by first radar device 86, whether only work Wor an object other than work W is in the first predetermined area.Furthermore, the distance between first radar device 86 and objectsdetermines the time taken until the reflected waves are observed (i.e.,the period t0˜t1 shown in FIG. 5D). Consequently, the time (t0˜t2) takenfor the reflected waves to be observed by first radar device 86 may bedetermined by the distance between first radar device 86 and the firstpredetermined area. As a result, it is acceptable for the time for firstradar device 86 to observe the reflected waves to be up until t2.

Next, second radar device 87 will be explained. FIG. 6 is a blockdiagram showing a representative circuit of the second radar 87. Asshown in FIG. 6, second radar device 87 may have antenna 104 fortransmitting and receiving radio waves. Oscillation circuit 102 isconnected to antenna 104 (specifically, to a radio wave transmittingmember of antenna 104), and clock circuit 100 is connected tooscillation circuit 102. Clock circuit 100 periodically transfers thefrequency of the signal that is output from oscillation circuit 102 totwo-phase, and also switches the state of switch 108. As a result, asshown in FIG. 7, the frequency of the signal that is output fromoscillation circuit 102 is periodically (1 period=2×ts) switched from ahigh frequency H to a low frequency L. Further, as the frequency of thesignal that is output from oscillation circuit 102 is switched, circuits(110 a˜114 a and 110 b˜114 b) for processing the signal from a radiowave receiving member of antenna 104 is simultaneously switched.Further, as is clear from FIG. 7, second radar device 87 differs fromfirst radar device 86, in that it continuously transmits radio waves atone of the two frequencies.

Moreover, diode mixer 106 is connected to antenna 104 (specifically, tothe radio wave receiving member of antenna 104). Diode mixer 106 is acircuit that mixes the radio waves received by antenna 104, that is, theradio waves that are transmitted from the radio wave transmitting memberof antenna 104 and the radio waves that have been reflected by areflector, and outputs these mixed waves (i.e., diode mixer 106 is aso-called waveform inspection circuit). The output from diode mixer 106changes on the basis of whether or not a reflector is moving towardssecond radar device 87. That is, if the reflector is not moving, theradio waves reflected by the reflector have the same frequency as theradio waves transmitted by antenna 104. On the other hand, due to theDoppler effect, if the reflector is moving, the radio waves reflected bythe reflector have a frequency different from that of the radio wavestransmitted by antenna 104. As a result, if the reflector is moving,radio waves having two close but differing frequencies mutuallyinterfere, causing beats to appear in the output waveform of diode mixer106. In second radar device 87 of the first representative embodiment,the frequency of these beats is used to measure the speed of movement ofthe reflector. Furthermore, the output from diode mixer 106 also differsfrom the frequency of the radio waves output from antenna 104. In thesecond radar device 87 of the first representative embodiment, the phasedifference of the beats produced by the two frequencies of the radiowaves created by the reflections from the reflector is used to measurethe position of the reflector (i.e., the distance from the second radardevice 87).

Two circuit groups are connected with diode mixer 106 via switch 108.That is, the first circuit group may comprise amplifying circuit 110 a,filter circuit 112 a and waveform shaping circuit 114 a. The secondcircuit group may comprise amplifying circuit 110 b, filter circuit 112b, and waveform shaping circuit 114 b. The first circuit group isconnected to diode mixer 106 while antenna 104 is transmitting radiowaves at the first frequency, and the second circuit group is connectedto diode mixer 106 while antenna 104 is transmitting radio waves at thesecond frequency. The structure and effects of the circuits is identicalwith the circuits used in first radar device 86.

The two waveform shaping circuits 114 a and 114 b are connected to phasedifference measuring circuit 118, whereas only waveform shaping circuit114 a is connected to speed measuring circuit 116. Phase differencemeasuring circuit 118 is a circuit for measuring the phase difference ofthe beats observed when the radio waves of both frequencies aretransmitted (in other words, measuring the distance of the reflector),and speed measuring circuit 116 is a circuit for measuring the phasedifference of the beats observed when the radio waves of the firstfrequency is transmitted (in other words, measuring the speed of thereflector). The output of phase difference measuring circuit 118 and ofspeed measuring circuit 116 are both output to control device 90.

Preferably, radio waves of 1 GHz or above may be used in the radio wavesoutput from second radar device 87; in the first representativeembodiment, 24.2 GHz microwaves may be used. This is because it ispreferred that second radar device 87 monitors only the surroundings ofcircular blade 3. In other words, as shown in FIG. 8, this is becausecontact with circular blade 3 is unlikely in locations at a distancegreater than a predetermined value (w/2 or greater) from side faces ofcircular blade 3. A further reason for using the above frequency is thatthe higher the frequency of radio waves the shorter the wavelength,which allows the location and speed of the reflector to be detectedaccurately. Moreover, the antenna shape and location of second radardevice 87 is determined so that the desired directivity (that is, adirectivity adequate to observe the second predetermined area) can beobtained when radio waves at the above frequencies are transmitted.

A representative circuit diagram for controlling table saw 1 will beexplained with reference to FIG. 9. As shown in FIG. 2, control device90, which disposed below table 5 (see FIG. 2), may include microcomputer92 and memory 94 (e.g., EEPROM). Microcomputer 92 may preferably includea CPU, ROM, RAM and I/O (interface), which are preferably integratedonto a single integrated circuit chip. The ROM of microcomputer 92 maystore programs for automatically stopping the driving operation of motorM. Memory 94 is connected to microcomputer 92 and stores the waveformsobserved by first radar device 86 when only work W is located in thefirst predetermined area near the outer edge of circular blade 3. Thereflected waveforms stored in memory 94 change each time the type (e.g.,thickness, wood type, etc.) of work W cut by table saw 1 changes.

First radar device 86 and second radar device 87 are connected tomicrocomputer 92, and the reflected waveforms output from first radardevice 86, and the speed and location of the reflector output fromsecond radar device 87 are input to the microcomputer 92. Power supplycircuit 98 is connected to motor M via driving circuit 96, and isconnected to microcomputer 92. Power supply circuit 98 is capable ofbeing connected to an external commercial power source, and supplies thepower supplied from this external commercial power source tomicrocomputer 92 and motor M. Further, motor switch 97 for startingmotor M is connected to microcomputer 92.

FIG. 10 shows a representative method for operating microcomputer 92 inorder to cut a work using table saw 1. That is, FIG. 10 is a flowchartof the process or program executed by microcomputer 92 during a cuttingoperation. In order to cut the work using the table saw 1, the operatorfirst turns a power switch ON, power supply to the microcomputer 92thereby beginning. At this time, motor switch 97 is OFF, consequentlycircular blade 3 does not begin to rotate.

When the power switch has been turned ON, as shown in FIG. 10,microcomputer 92 waits until motor switch 97 is turned ON (step S10).The operator first positions the work in the first predetermined area(i.e., the anterior of circular blade 3), then turns the motor switch 97ON. When motor switch 97 has been turned ON (YES in step S10),microcomputer 92 causes first radar device 86 to operate, and receivesthe waveforms of the signals that are output from first radar device 86(step S12). The received waveforms are the reflected waveforms from theradio waves reflected from the work. When the waveforms of the signalsoutput from first radar device 86 have been received, microcomputer 92stores these received waveforms in memory 94 (step S14).

Further, when motor switch 97 has been turned ON (YES in step S16),microcomputer 92 outputs a ON signal to driving circuit 96, thisstarting the supply of power to motor M from power circuit 98, andsimultaneously causing the operation of first radar device 86 and secondradar device 87. As a result, circular blade 3 begins to rotate, and themeasured results from first radar device 86 and second radar device 87are periodically output. Microcomputer 92 first reads in the output(i.e., the speed and location of the object moving within the secondpredetermined area) from second radar device 87 (step S18).

Then, microcomputer 92 determines whether the distance from second radardevice 87 to the object, which was read in in step S18, is equal to orgreater than a predetermined value 1 (step S20). This predeterminedvalue 1 is shorter than the distance from second radar device 87 tocircular blade 3. If the measured distance is below the predeterminedvalue 1 (NO in step S20), microcomputer 92 quickly stops motor M (stepS30). Specifically, microcomputer 92 outputs an OFF signal to drivingcircuit 96, this cutting off the supply of power to motor M. By thismeans, the rotation of motor M is halted.

As described above, the driving operation of motor M is halted when thedistance measured by second radar device 87 is below the predeterminedvalue 1 (that is, when an object is between second radar device 87 andcircular blade 3). Motor M is halted in this manner because objectsextremely close to second radar device 87 prevent second radar device 87from monitoring the surroundings of circular blade 3.

If the measured distance is equal to or greater than the predeterminedvalue 1 (YES in step S20), microcomputer 92 determines whether thedistance from second radar device 87 to the object, which was read in instep S18, is equal to or less than a predetermined value 2 (step S22).This predetermined value 2 is greater than the predetermined value 1,and is longer than the distance from second radar device 87 to circularblade 3. If the measured distance exceeds the predetermined value 2 (NOin step S22), the process proceeds to step S26. On the other hand, ifthe measured distance is equal to or below the predetermined value 2(YES in step S22), microcomputer 92 determines whether the speed of theobject read in in step S18 is equal to or less than a predeterminedspeed (step S24). If the speed of the object read in in step S18 isequal to or less than the predetermined speed (YES in step S24), theprocess proceeds to step S26. If the speed of the object read in in stepS18 exceeds the predetermined speed (NO in step S24), microcomputer 92quickly stops motor M (step S30).

Thus, in the case where the object measured by second radar device 87 iswithin zone I shown in FIG. 11, (that is, in the case where the distancefrom second radar device 87 is below the predetermined value 1), thedriving operation of motor M is halted. In the case where the objectmeasured by second radar device 87 is within zone II (that is, in thecase where the distance from second radar device 87 is equal to or abovethe predetermined value 1 and equal to or less than the predeterminedvalue 2), motor M is halted only when the speed of the object exceeds apredetermined speed. Further, in the case where the object measured bysecond radar device 87 is within zone III (that is, in the case wherethe distance from second radar device 87 exceeds the predetermined value2), motor M is not halted since the likelihood of contact with circularblade 3 is low.

Microcomputer 92 proceeds to step S26 and takes up the output waveformsfrom first radar device 86. Then, microcomputer 92 determines whetherthe absolute value of the difference between the peak values of theoutput waveforms taken up in step S8 (that is, the peak values of thereflected waves reflected from the object in the first predeterminedarea) and the peak values of the output waveforms stored in memory 94 instep S2 (that is, the peak values of the reflected waves reflected fromthe work in the first predetermined area) is equal to or below apredetermined value 3 (step S28).

If the absolute value of the difference between the peak values of thetwo output waveforms is equal to or below the predetermined value 3 (YESin step S28), microcomputer 92 determines that an object other than workis not present in the first predetermined area, and returns to step S16.Consequently, if motor switch 97 is in an ON state (YES in Step S16),the process after step S18 is repeated. As a result, the rotation ofcircular blade 3 continues while being monitored by first radar device96 and second radar device 87, and the operator can perform the cuttingoperation by sending the work from the anterior at a safe speed.

On the other hand, if the absolute value of the difference between thepeak values of the two output waveforms exceeds the predetermined value3 (NO in step S28), microcomputer 92 determines that an object otherthan work is present in the first predetermined area, and stops thedriving operation of motor M (step S30).

In summary, in the table saw of the first representative embodiment, thesurroundings of circular blade 3 are monitored by second radar device87, and the vicinity of the outer edge of circular blade 3 is monitoredby first radar device 86, this allowing the possibility of contactbetween circular blade 3 and an object other than work to be detectedbefore this contact is made, and halting the driving operation of motorM. As a result, it is possible to prevent contact between the objectother than work and the rotating circular blade 3.

Moreover, only radio waves of a single frequency are transmitted fromfirst radar device 86 and second radar device 87. Consequently, antennas124 and 104 for receiving the reflected waves can be compact, and it ispossible to simplify, for example, the amplifying circuit for amplifyingthe received reflected waves.

Moreover, in the table saw of the first representative embodiment, theuse of blade guard 7 allows the monitored area near circular blade to berestricted, thus limiting the number of radar devices. In other words,by using blade guard 7, all that is monitored is the movement, in thedirection in which work is sent, of objects near the circular blade, andonly the area near the outer edge of the circular blade is monitored. Asa result, operation becomes safer using by means of both blade guard 7and first radar device 86 and second radar device 87.

Further, in the first representative embodiment, second radar device 87is attached to the tip of the arm attached to table 5. However, secondradar device 87 is not restricted to this type of configuration. Forexample, second radar device 87 may be disposed according to the methodsshown in FIGS. 12A-12C. In FIG. 12A, arm 85 is attached to the lowerportion of the table saw, second radar device 87 being attached to thetip of arm 85. Further, FIGS. 12B and 12C show cases where the table sawis fixed to a floor. In FIG. 12B, arm 85 is fixed to a wall to theposterior of the table saw and second radar device 87 is attached to thetip of arm 85, and in FIG. 12C, arm 85 is fixed to a ceiling and secondradar device 87 is attached to the tip of arm 85.

Further, in the first representative embodiment, motor M immediatelyhalts when the results measured by first radar device 86 and secondradar device 87 fulfill predetermined conditions. However, aconfiguration is also possible wherein decision criteria are set at twostages; first, the operator is warned when the first stage of thedecision criteria is exceeded, then the driving operation of thecircular blade is halted when the second stage of the decision criteriais exceeded. For example, the region to the anterior of circular blade 3in zone II of FIG. 11 is divided into a further two regions. If it isdetermined that an object is anomalously in the region further fromcircular blade 3, the warning is sounded, and if it is determined thatan object is anomalously in the region closer to circular blade 3, anemergency stop of the motor is performed. With this type ofconfiguration, the operator can be alerted by the warning, thus avoidinginterruptions to the cutting operation.

Moreover, in the first representative embodiment, single frequency radiowaves are transmitted from first radar device 86. However, it is alsopossible that first radar device 86 transmits radio waves that includeall frequencies, such as impulses, and analyzes the frequencies of thereflected waves to more precisely identify objects in the firstpredetermined area.

Furthermore, in the first representative embodiment, motor M halts whenit is determined that there is a likelihood of contact occurring betweencircular blade 3 and objects other than work. However, it is alsopossible to provide a retracting mechanism whereby the circular blade isretracted from above to below the table at times of emergency, or toprovide a brake mechanism that engages and stop the circular blade attimes of emergency.

Second Detailed Representative Embodiment

The table saw of the second representative embodiment has substantiallythe same configuration as the table saw of the first representativeembodiment, differing only in using a microstrip antenna in place of theantenna 104 of second radar device 87 of the first representativeembodiment. Consequently, in the following description only the pointsdiffering from the first representative embodiment will be explained.

First, the configuration of the microstrip antenna will be explainedwith reference to FIGS. 13A-13G. As shown in FIG. 13A, microstripantenna 130 a may comprise strip line 132 a, dielectric substrate 134 a,and flat conductor 136 a. Flat conductor 136 a may have an area greaterthan strip line 132 a. In the case where a body (e.g., a table of atable saw) of a power tool is formed from a conductive material (e.g., ametal plate made from aluminum), the body may be used as the flatconductor 136 a. Flat conductor 136 a is connected to a ground. Further,flat conductor 136 a need not necessarily be flat. Dielectric substrate134 a may be disposed on a surface of flat conductor 136 a. Dielectricsubstrate 134 a is a plate-shaped dielectric substance that utilizes,for example, teflon resin, fiberglass epoxy resin, or the like. Inparticular, in the case where the frequency of radio waves to betransmitted and received is 1 GHz or above, teflon resin is preferablyutilized. The thickness of the dielectric substrate 134 a may be, forexample, up to 1 mm. Strip line 132 a may be disposed on a surface 134 sof dielectric substrate 134 a. Strip line 132 a may be formed from aconductive material, such as, for example, copper foil (thickness up to35 μm). Strip line 132 a is connected to a feeder line.

When signals are input to strip line 132 a from an oscillation circuit,the voltage between strip line 132 a and flat conductor 136 afluctuates. By this means, radio waves are transmitted between stripline 132 a and flat conductor 136 a. The transmitted radio waves aresent to the surface 134 s of dielectric substrate 134 a. Thus,microstrip antenna 130 a may be disposed on the power tool such that theobjects to be measured approach the surface 134 s of dielectricsubstrate 134 a. For example, microstrip antenna 130 a may be disposedon a surface of the power tool opposite the objects to be measured.

Preferably, the radio waves transmitted from microstrip antenna 130 amay be approximately 1 GHz or above. For example, 24.2 GHz microwavesmay be used. The reason is that having the radio waves at a higherfrequency improves the directivity thereof, allowing the objects to bemeasured to be detected with greater accuracy. Furthermore, thefrequency of the radio waves transmitted from microstrip antenna 130 amay be modified so as to be adapted to the properties of the objects tobe measured.

In the example shown in FIG. 13A, strip line 132 a is copper foil and,due to a surface thereof protruding, may be damaged by abrasion. In thiscase, it is preferred that microstrip antenna 130 a may be disposedwithin a housing of the power tool. Further, the housing may include apenetrable window through which the radio waves transmitted frommicrostrip antenna 130 a penetrate.

FIGS. 13B˜13G show another example of microstrip antennas. The exampleshown in FIG. 13B utilizes strip conductor 132 b in place of strip line132 a in FIG. 13A. Strip conductor 132 b may be formed from a conductivematerial (e.g., a metal plate made from aluminum). The use of stripconductor 132 b increases the strength thereof against abrasion orimpact. In this case, it is preferred that microstrip antenna 130 b maybe disposed on the surface of the power tool. Furthermore, microstripantenna 130 b may have a certain degree of thickness (e.g., up to 1 mm).As a result, it is possible to form a groove in dielectric substrate 134b and to dispose strip conductor 132 b within this groove. When stripconductor 132 b is in a disposed state within the groove, it ispreferred that a surface of strip conductor 132 b extends along the sameplane as a surface of dielectric substrate 134 b.

In the example shown in FIG. 13C, dielectric substrate 134 c does nothave a thickness sufficient to provide a groove therein. Consequently,the portions of dielectric substrate 134 c not having strip conductor132 c disposed thereon may have a filling material 138 c disposedthereon. Filling material 138 c allows a surface of strip conductor 132c and a surface of Filling material 138 c to extend along one plane.Filling material 138 c may be preferably an insulating material, and amaterial with little dielectric loss. Filling material 138 c may beformed from, for example, resin, cement, or the like.

Further, in cases where it is not desirable to provide a width like thatof dielectric substrate 134 b in the example shown in FIG. 13B, or awidth like that of filling member 138 c in the example shown in FIG.13C, configurations like those shown in FIGS. 13D and 13E are alsopossible. That is, in the example shown in FIG. 13D, a groove may beformed in flat conductor 136 d, and dielectric substrate 134 d and stripconductor 132 d may be disposed within the groove. By this means, thearea of a surface of dielectric substrate 134 d can be reduced.Similarly, in the example shown in FIG. 13E, flat conductor 136 e mayhave a groove, dielectric substrate 134 e and strip conductor 132 e maybe disposed within the groove, and remaining portions may be filled withfilling material 138 e.

Moreover, the configurations shown in FIGS. 13F and 13G are alsopossible. In the examples shown in FIGS. 13F and 13G, side walls of flatconductors 136 f and 136 g are inclined faces 137 f and 137 g. In thiscase, the radio waves that are transmitted are easily delivered at theside with inclined faces 137 f and 137 g, and a desirableelectromagnetic field (i.e., detecting area) can be formed.

The microstrip antennas configured as described above may be disposed ina table surface of the table saw. FIG. 14 shows an example of anarrangement wherein a microstrip antenna is disposed in a surface oftable 144. Located in the surface of table 144 shown in FIG. 14 are: atransmitting and receiving device 152 for transmitting and receivingradio waves; and a plurality of microstrip antennas or patch antennas154 a˜154 d (hereafter referred to simply as patch antennas).Transmitting and receiving device 152 fulfils the functions of thecircuits 100, 102, 106, 108, 110 a, 110 b, 112 a, 112 b, 114 a, 114 b,116, and 118 shown in FIG. 6. Transmitting and receiving device 152 maybe disposed to the posterior (i.e., the direction opposite the operatorside) of circular blade 142. Patch antennas 154 a˜154 d are a type ofmicrostrip antenna and fulfill the functions of antenna 104 shown inFIG. 6. Two each of the patch antennas 154 a˜154 d may be disposed onleft and right sides of circular blade 142, being separated from oneanother in an anterior-posterior direction.

FIG. 15 is a cross-sectional view of patch antenna 154 a. As shown inFIG. 15, patch antenna 154 a comprises strip or patch 156 (hereafterreferred to simply as patch), dielectric substrate 158, and table 144.That is, patch 156 corresponds to the strip conductor of FIGS. 13A-13G,dielectric substrate 158 corresponds to the dielectric substrate ofFIGS. 13A-13G, and table 144 corresponds to the flat conductor of FIGS.13A-13G.

A groove is formed in table 144, and dielectric substrate 158 isdisposed within this groove. Further, a groove is formed in dielectricsubstrate 158, and patch 156 is disposed within this groove. As is clearfrom FIG. 15, surfaces of table 144, dielectric substrate 158, and patch156 all extend along one plane. As a result, patch 156 or dielectricsubstrate 158 do not form an obstruction when the work is slid acrossthe table 144. Moreover, by being disposed within table 144, patchantenna 154 a does not obstruct a design where mechanisms are disposedbeneath table 144 (e.g., a inclining mechanism for inclining circularblade 142, etc.). Further, remaining patch antennas 154 b, 154 c, and154 d may have the same configuration as patch antenna 154 a describedabove.

As shown in FIG. 14, transmitting and receiving device 152 and patchantennas 154 a˜154 d are connected with a feeder line L. Feeder line Lmay include two phase shifters 156 a. That is, one of phase shifters 156a is disposed between patch antenna 154 a and patch antenna 154 c, andother phase shifter 156 a is disposed between patch antenna 154 b andpatch antenna 154 d. By this means, as shown in the figure on the rightin FIG. 14, the transmitting and receiving direction of the radio wavesof patch antennas 154 a˜154 d is altered towards the operator. As aresult, radar device 150 can monitor objects to be measured that move inthe area surrounding circular blade 142 protruding above table 144(particularly the area towards the operator). Furthermore, thedimensions, number, location, etc. of patch antennas 154 a˜154 d may beadapted to correspond to the characteristics of the objects to bemeasured.

As is clear from the above description, using the microstrip antennaallows the antenna to be miniaturized, and allows the antenna to bedisposed in the surface of the power tool. By this means, a greaterdegree of design freedom can be obtained concerning the location of theantenna.

The second representative embodiment described above can be embodiedwith a variety of transformations or improvements thereto. For example,in the example shown in FIG. 16, transmitting device 170 is disposed tothe posterior of circular blade 142 and receiving device 176 is disposedto the anterior of circular blade 142. Transmitting device 170 mayinclude transmitting machine 174 and patch antennas 172 a and 172 b,these being connected via a feeder line L. Further, receiving device 176may include receiving machine 180 and patch antennas 178 a and 178 b,these being connected via a feeder line L. This type of configurationallows the detection of objects to be measured between transmittingdevice 170 and receiving device 176 (that is, in the vicinity ofcircular blade 142).

Further, as shown in FIG. 17, it is also possible to locate transmittingand receiving device 184 to the posterior of circular blade 142, and tolocate patch antennas 186 a˜186 c, and 188 a˜188 c to the left and rightsides respectively of circular blade 142. In other words, the location,number, etc. of the patch antennas can be varied. Moreover, in thesecond representative embodiment, the microstrip antenna is used in theantenna of a radar device(corresponding to second radar device 87 of thefirst representative embodiment) that detects the objects to be measuredby means of Doppler radar. However, the microstrip antenna may be usedin a different type of radar (for example, first radar device 86 in thefirst representative embodiment).

Although the first and the second representative embodiment have beendescribed in terms of a table saw, the present teachings can naturallybe applied to other power tools, such as a miter saw, a slide-type tablesaw, a slide-type circular saw, etc.

Further, a detecting device which performs radio wave sensing by meansof a microstrip antenna have been described in detail above. However,this type of detecting device can also be applied to the power toolsdescribed below.

The detecting device described above can also be applied to a demolitionhammer. During operation, the vibration of a demolition hammer causesthe vibration of not only the tool, but also of the operator's body. Inparticular, if the vibration is great, the head of the operator is alsocaused to vibrate. On the other hand, the force with which the hammerstrikes the work can be reduced, thereby decreasing the vibrationtransmitted to the operator; however, in this case, operating efficiencyfalls as the force with which the hammer strikes the work is reduced. Todeal with this problem, the vibration, etc. being transmitted to theoperator's head can be detected by means of the detecting device, and astructure can be formed for canceling the vibration. Specifically, thedemolition hammer may include a counter-balance and a cancelingmechanism for canceling the vibration transmitted to the operator viathe counter-balance. The demolition hammer may further include thedetecting device which, by means of transmitting radio waves towards theoperator, detects the movement of the operator relative to the hammer.Doppler radar, for example, can be used as the radio wave sensingmethod. Further, an antenna (e.g., a microstrip antenna) of thedetecting device can be disposed in a location from where the radiowaves can be transmitted towards the operator. For example, the antennamay be disposed within an upper face of a housing. The demolition hammermay further include a control device that can control the cancelingmechanism in response to the vibration of the operator's head, thevibration having been detected by the detecting device. Moreover, a pickup may be disposed separately within the housing, measured values fromthis pick up and the detected values from the detecting device beingcompared, and the counter-balance being adjusted appropriately.

The detecting device described above can be applied to a jig saw. Thejig saw cuts wood by pressing the wood against an inner face of a shoeand moving the jig saw while the wood is in this state. The cutting loadvaries according to the moisture content and thickness of the wood.Accordingly, the moisture content and thickness of the wood can bedetected by means of the detecting device and the detected values usedas feedback for the rotation speed of a motor, thereby improving cuttingoperation. Specifically, a microstrip antenna may be disposed within theinner face (preferably, in a cutting direction viewed from saw blade) ofthe shoe. The method of radio wave sensing may be, for example, a pulsemethod whereby radio waves are transmitted in pulses, and the reflectedwaves therefrom are received. A control device may determine themoisture content or the thickness of the work on the basis of peakvalues of the reflected waves received by the microstrip antenna. Thecontrol device then controls the rotation speed of the motor inaccordance with this moisture content and thickness. Furthermore, themoisture content and thickness may be displayed to the operator by meansof an indicator or the like. Further, in the case where the saw blade ison the point of cutting the support for the work, or foreign materialssuch as nails etc. are discovered, a warning may be given and the motorhalted.

The detecting device described above can be utilized for preventing thetheft of power tools (e.g., a compressor). That is, a microstrip antennacan be disposed within an upper face of a housing of the compressor.Doppler radar, for example, can be used as the method of radio wavesensing. Power for the microstrip antenna can be supplied from a batterythat can be removably attached to the compressor. If a person approachesthe compressor, or tries to move the compressor, this is detected by themicrostrip antenna, an alarm is sounded, and the compressor is disabled.By this means, the theft of the compressor can be prevented. On theother hand, the owner of the compressor carries a transmitter. When thecompressor receives radio waves transmitted from this transmitter, thealarm is not sounded, and the compressor is not disabled.

Finally, although the preferred representative embodiment has beendescribed in detail, the present embodiment is for illustrative purposeonly and not restrictive. It is to be understood that various changesand modifications may be made without departing from the spirit or scopeof the appended claims. In addition, the additional features and aspectsdisclosed herein also may be utilized singularly or in combination withthe above aspects and features.

1. A power tool, comprising: a cutting tool; a motor for driving thecutting tool; means for detecting the location of objects moving withina predetermined area in the vicinity of the cutting tool and fordetecting the speed of approach of the objets towards the cutting tool;and a processor in communication with the detecting means, wherein theprocessor determines whether the object detected by the detecting meanshas a predetermined positional relationship relative to the cutting tooland determines whether the detected speed exceeds a predetermined valuewherein the processor stops the motor when the processor determines thatthe object detected by the detecting means has the predeterminedpositional relationship relative to the cutting tool and that thedetected speed exceeds the predetermined value.
 2. A power tool as inclaim 1, further comprising a table, wherein a portion of the cuttingtool protrudes above the table, wherein the cutting tool cuts the workpositioned on an upper face of the table.
 3. A power tool as in claim 1,wherein the detecting mean comprises a radar for transmitting radiowaves towards the predetermined area and for receiving waves reflectedtherefrom.
 4. A power tool as in claim 3, wherein the radar is disposedin a position such that the cutting tool is sandwiched therebetween, andsuch that the radar faces the operator.
 5. A power tool as in claim 3,wherein the frequency of the radio waves transmitted from the radar is 1GHz or above.
 6. A power tool as in claim 5, wherein the frequency ofthe radio waves transmitted from the radar is within the range of 10˜30GHz.